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.Dd December 2, 2023
.Dt GETGROUPLIST 3C
.Os
.Sh NAME
.Nm getgrouplist
.Nd calculate group access list
.Sh SYNOPSIS
.In grp.h
.Ft int
.Fo getgrouplist
.Fa "const char *user"
.Fa "gid_t agroup"
.Fa "gid_t *groups"
.Fa "int *ngroups"
.Fc
.Sh DESCRIPTION
The
.Fn getgrouplist
function queries the group database to obtain the list of groups that
.Fa user
belongs to.
The
.Fa agroup
group is always added to the resulting group list.
This value is typically the primary gid of the user from the
.Sy passwd
database.
.Pp
When calling
.Fn getgrouplist ,
the caller should set the maximum number of groups that
.Fa groups
can hold in
.Fa *ngroups .
The value of
.Dv NGROUPS_MAX
can be used to size
.Fa groups
to ensure it can hold any number of groups supported by the system.
.Pp
Upon return,
.Fn getgrouplist
stores the list of groups that
.Fa user
belongs to in
.Fa groups
and stores the number of groups
.Fa user
belongs to in
.Fa *ngroups
.Po
this may be a smaller than the value passed in when
calling
.Fn getgrouplist
.Pc .
If
.Fa groups
is too small to hold all of the groups
.Fa user
belongs to,
.Fn getgrouplist
fails and sets
.Fa *ngroups
to a value large enough to hold the full result.
.Sh RETURN VALUES
On success,
.Fn getgrouplist
returns the number of groups
.Fa user
belongs to, fills in
.Fa groups
with the gids of the groups
.Fa user
belongs to, and also sets
.Fa *ngroups
to the number of groups
.Fa user
belongs to.
.Pp
On failure,
.Fn getgrouplist
returns -1 and
.Va errno
is set.
.Pp
The behavior of
.Fn getgrouplist
is undefined if the total number of groups a user belongs to exceeds
.Dv NGROUPS_MAX .
.Pp
Note that on
.Fx ,
.Fn getgrouplist
always returns -1 on failure or 0 on success.
A caller must rely on the value set in
.Fa *ngroups
upon return to determine the number of entries in
.Fa groups .
.Pp
On Linux, both glibc and musl return the number of groups
.Fa user
belongs to on success and return -1 on failure.
.Pp
None of these other implementations document any
.Va errno
values on failure, however their implementations show that
.Va errno
may be set on failure.
Software using
.Fn getgrouplist
should be aware of these differences when attempting to write portable
software.
.Sh EXAMPLES
.Sy Example 1
Print all the groups for a user.
.Bd -literal
#include <pwd.h>
#include <grp.h>
#include <stdlib.h>
#include <stdio.h>
#include <limits.h>
#include <err.h>

void
printgroups(const char *user)
{
    struct passwd *pw;
    gid_t *groups;
    int ngroups, ret;

    if ((groups = calloc(NGROUPS_MAX, sizeof (gid_t))) == NULL)
        err(EXIT_FAILURE, "calloc");

    if ((pw = getpwnam(user)) == NULL)
        err(EXIT_FAILURE, "getpwnam");

    ngroups = NGROUPS_MAX;
    ret = getgrouplist(user, pw->pw_gid, groups, &ngroups);
    if (ret < 0)
        err(EXIT_FAILURE, "getgrouplist");

    for (int i = 0; i < ret; i++) {
        struct group *gr = getgrgid(groups[i]);

        (void) printf("%s ", gr->gr_name);
    }
    (void) fputc('\en', stdout);

    free(groups);
}
.Ed
.Sh ERRORS
On failure,
.Fn getgrouplist
returns -1, and will set errno to one of the following values:
.Bl -tag -width Dv
.It Er ENOMEM
Not enough memory to complete the request.
.It Er EINVAL
One of the parameters is invalid
.Po
for example,
.Fa ngroups
is
.Dv NULL
.Pc .
.It Dv ERANGE
The supplied value of
.Fa *ngroups
is too small to hold the results.
.Fa *ngroups
is set
.Po
upon return
.Pc
to a value large enough to hold the results, and a partial set of
results is written to
.Fa groups .
The value written to
.Fa *ngroups
may be larger than the value returned by a successful call to
.Fn getgrouplist .
.El
.Sh INTERFACE STABILITY
.Sy Uncommitted
.Sh MT-LEVEL
.Sy MT-Safe
.Sh SEE ALSO
.Xr groups 1 ,
.Xr getgroups 2 ,
.Xr getuid 2 ,
.Xr getgrnam 3C ,
.Xr initgroups 3C ,
.Xr limits.h 3HEAD
